1. Field of the Invention
The present invention relates to high performance semiconductor memory devices, and more particularly to embedded memory devices having first level bit lines connected along different layout directions.
2. Description of the Prior Art
DRAM is usually considered as a high density, low cost, but low performance memory device. DRAM's of current art always have lower performance relative to other types of semiconductor memories such as static random access memory (SRAM). The density of DRAM has been improved rapidly; the extent of integration has been more than doubled for every generation. Such higher integration of DRAM has been realized mainly by super fine processing technique and improvements in memory cell structure. In the mean time, the improvement in DRAM performance is progressing at a much slower rate. This relatively slower improvement rate in performance generates a performance gap between logic devices and memory devices. Many new approaches have been proposed to reduce this performance gap. The synchronized DRAM (SDRAM), the extended data output (EDO) DRAM, the multiple bank DRAM (MDRAM), and the RAMBUS system approaches are the most well known methods to improve DRAM performance. U.S. Pat. No. 4,833,653 issued to Mashiko et al. and U.S. Pat. No. 4,758,993 issued to Takemae et al. disclosed DRAM having selectively activated subarrays in order to improve performance. Another approach to improve DRAM performance is to place an SRAM cache into DRAM (called "hybrid memory"). U.S. Pat. No. 5,421,000 issued to Fortino et al., U.S. Pat. No. 5,226,147 issued to Fujishima et al., U.S. Pat. No. 5,305,280 issued to Hayano et al. disclosed embodiments of hybrid memories. The major problem for above approaches is that they are paying very high price for performance improvement, while the resulting memory performance improvement is still not enough to fill the gap. Another problem is that all of those approaches require special system design that is not compatible with existing computer systems; it is therefore more difficult to use them in existing computer systems.
Another disadvantage of DRAM is the need to refresh its memory. That is, the users need to read the content of memory cells and write the data back every now and then. The system support for DRAM is more complex than SRAM because of this memory refresh requirement Memory refresh also represents a waste in power. U.S. Pat. No. 5,276,843 issued to Tillinghast et al. disclose a method to reduce the frequency of refresh cycles. U.S. Pat. No. 5,305,280 issued to Hayano et al. and U.S. Pat. No. 5,365,487 issued to Patel et al. disclosed DRAM's with self-refresh capability. Those inventions partially reduce power consumption by refresh operations, but the magnitude of power saving is very far from what we can achieve by the present invention. The resource conflict problem between refresh and normal memory operations also remains unsolved by those patents.
Recently, Integrated Device Technology (IDT) announced that the company can make DRAM close to SRAM performance by cutting DRAM into small sub-arrays. The new device is not compatible with existing memory; it requires special system supports to handle conflicts between memory read operation and memories refresh operation. It requires 30% more area the DRAM, and its performance is still worse than SRAM of the same size.
Another important problem for DRAM design is the tight pitch layout problem of its peripheral circuits. In the course of the rapid improvement in reducing the size of memory cells, there has been no substantial improvement or change as to peripheral circuits. Peripheral circuits such as sense amplifiers, decoders, and precharge circuits are depend upon memory cell pitch. When the memory cells are smaller for every new generation of technology, it is more and more difficult to "squeeze" peripheral circuits into small pitch of memory layout. This problem has been magnified when the memory array is cut into smaller sub-arrays to improve performance. Each subarray requires its own peripheral circuits; the area occupied by peripheral circuits increases significantly. Therefore, in the foreseeable future, there may occur a case wherein the extent of integration of DRAM is defined by peripheral circuits. U.S. Pat. No. 4,920,517 issued to Yamauchi et al. disclosed a method to double the layout pitch by placing sense amplifiers to both ends of the memory. This method requires additional sense amplifiers. Although the available layout pitch is wider than conventional DRAM, the layout pitch is still very small using Yamauchi's approach.
All of the above inventions and developments provided partial solutions to memory design problems, but they also introduced new problems. It is therefore highly desirable to provide solutions that can improve memory performance without significant degradation in other properties such as area and user-friendly system support.
Another difficulty encountered by those of ordinary skill in the art is a limitation that Dynamic Random Access Memory (DRAM) which is usually considered as a high density, low cost, and low performance memory device cannot be conveniently integrated as embedded memory. This is due to the fact that higher integration of DRAM has been realized mainly by super fine processing technique and improvements in memory cell structure. A typical DRAM manufacture technology of current art is the four layer poly silicon, double layer metal (4P2M) process. Such memory technology emphasizes on super-fine structure in manufacture memory cells; performance of it logic circuit is considered less important. A technology optimized to manufacture high speed logic products have completely different priority; it emphasizes on performance of transistors, and properties of multiple layer metals. An example of a typical logic technology of current art is the triple layer metal, single poly silicon (1P3M) technology.
An embedded memory, by definition, is a high density memory device placed on the same chip as high performance logic circuits. The major challenge to manufacture high density embedded memory is the difficulty in integrating two types of contradicting manufacture technologies together. An embedded technology of current art requires 4 layers of poly silicon and 3 layers of metal. There are more than 20 masking steps required for such technology. It is extremely difficult to have reasonable yield and reliability from such complex technology of current art. Further more, the current art embedded technology tend to have poor performance due to contradicting requirements between logic circuits and memory devices. None of current art embedded memory technology is proven successful. There is an urgent need in the Integrated Circuit (IC) industry to develop successful embedded memory devices.
The Applicant of this Patent Application has been successful in manufacturing embedded memory devices by novel approaches to change the architecture of IC memory so that the memory device no longer has conflicting properties with logic circuits. Examples of such architecture change have been disclosed in co-pending patent application Ser. No. 08/653,620. The previous application solved the tight pitch layout problems along the sense amplifier location, and it solves the self-refresh requirement by hiding refresh requirements. This CIP Application further discloses solutions for remaining problems. A single-transistor decoder circuit solves the tight pitch layout problem along the decoder direction. Typical logic technology or small modification of existing logic technology may be applied to manufacture the memory cells. Using these novel inventions, high performance and high density embedded memory devices are ready to be manufactured.